An optically active carboxylic acid of the formula (4): EQU R.sub.1 --COS--(CH.sub.2)n--CH(R.sub.2)--COOH (4)
wherein R.sub.1 is alkyl, aralkyl or aryl, R.sub.2 is alkyl, and n is 1 or 2, is a valuable starting material for the production of various optically active compounds having physiological activities. For example, D(-)-.beta.-acetylthioisobutyric acid is an important intermediate for the preparation of N-(D-.alpha.-methyl-.beta.-mercaptopropionyl)-L-proline which is useful hypotensive agent of angiotensin converting enzyme inhibitor series.